1. Technical Field
The invention is related to RF plasma etch reactors capable of high selectivity between etching of oxide films (such as silicon dioxide) and non-oxide surfaces (such as polysilicon or silicon nitride films).
2. Background Art
European Patent Document No. 0,520,519 A1 discloses a novel inductively coupled plasma etch reactor for etching thin films formed on a silicon wafer using an RF plasma etch process, the disclosure of which is incorporated herein by reference. Such a reactor can be used to selectively etch silicon dioxide over non-oxide (e.g., silicon nitride) films on the wafer. Specifically, a carbon-fluorine gas such as C.sub.2 F.sub.6 is excited sufficiently to generate a plasma within the reactor chamber, producing ions and free radicals including F and CF.sub.3, for example. The F radicals etch any silicon dioxide film on the wafer, while carbon and fluorine atoms or ions in the plasma combine on the wafer surface to form a polymer. The polymer disassociates when formed on silicon dioxide surfaces due to the effect of oxygen freed from the silicon dioxide film during the etch process, and due to effect of fluorine in the plasma. However, when formed on a non-oxide film (e.g., silicon nitride), the polymer accumulates due to the lack of oxygen in the underlying non-oxide film, this formation inhibiting etching of the underlying non-oxide film, thereby providing a pronounced etch selectivity of the oxide film over the non-oxide film. This selectivity is of great advantage when etching vias through a silicon dioxide layer overlying a non-oxide layer (e.g., polysilicon) which is not to be etched. The selectivity is limited if the polymer formed over the polysilicon layer contains more than 40% fluorine by weight, because such polymers are susceptible to being attacked by fluorine in the plasma, and therefore provide only limited protection to the underlying polysilicon layer.
U.S. patent application Ser. No. 07/941,501 filed Sep. 8, 1992 by Marks et al. entitled "Selectivity for Etching an Oxide Over a Nitride" discloses how to use an inductively coupled plasma reactor of the type disclosed in the above-referenced European Patent Document to form a carbon polymer film having less than 40% fluorine over a non-oxide (i.e., silicon nitride) film. This improvement is realized by increasing the proportion of carbon in the plasma relative to fluorine, and is accomplished by introducing a fluorine scavenger into the plasma. One such scavenger is silane gas, for example. The silicon in the silane gas combines with free fluorine atoms in the plasma to form SF.sub.4 gas, which is readily pumped out of the reactor chamber. The effect of this improvement is that the carbon-rich polymer formed over the silicon nitride film is impervious to fluorine in the plasma and thereby provides a virtually infinite selectivity of silicon dioxide etch rate to silicon nitride etch rate.
U.S. patent application Ser. No. 07/984,045 filed Dec. 1, 1992 by Collins et al. and U.S. patent application Ser. No. 07/941,507 filed Sep. 8, 1992 by Collins et al. disclose, respectively, a capacitively and an inductively coupled plasma etch apparatus in which a fluorine scavenger material is introduced into the reactor chamber to achieve the same type of advantages as realized in the above-referenced Marks et al. application. This material is in the form of a silicon ceiling inside the reactor chamber. The silicon ceiling emits silicon atoms into the plasma which scavenge fluorine out of the plasma, providing the desired carbon-to-fluorine ratio in the plasma to form a carbon-rich polymer impervious to fluorine in the plasma over the non-oxide (e.g., silicon nitride) film.
The problem with the foregoing techniques is that in many types of reactors, particularly the inductively coupled reactors of the type disclosed in the above-referenced European Patent Document, the chamber side walls are preferably formed of quartz (silicon dioxide) because the silicon dioxide atoms on the wall surface etch to provide silicon and oxygen atoms. The silicon atoms scavenge fluorine out of the plasma, with the desired effects described above. To a lesser extent, the oxygen atoms combine with carbon atoms in the plasma to scavenge carbon, but this is a minor effect.
The quartz side walls are susceptible to cooling each time the reactor is idled and the plasma turned off, which is typical whenever a new cassette of wafers is introduced or whenever the chamber must be opened for maintenance, for example. The side walls typically fall below 170 degrees C., which is the deposition temperature below which the carbon-fluorine polymer condenses. As soon as the plasma is ignited again, the carbon-fluorine polymer formed from the plasma condenses very rapidly onto the now-cool quartz side walls, forming a very thick polymer coating. As each wafer is cycled through the chamber, the temperature of the side wall climbs, dropping slightly and temporarily as the plasma is turned off briefly with the introduction of each new wafer, but making steady overall progress upward to its stead-state temperature, as illustrated in FIG. 1. In the meantime, because the quartz side wall is attracting so much of the polymer, very little is available to protect the non-oxide (e.g., polysilicon or silicon nitride) film on the wafer from being etched, and so the oxide-to-non-oxide etch selectivity is below the required level during the processing of the first several wafers.
Eventually, the temperature of the quartz side wall climbs above 170 degrees C., and the thick polymer coating suddenly vaporizes from the now-hot side wall and covers the new wafer currently being processed in the reactor chamber, interfering with the etch process.
One way around the foregoing problem is to delay the introduction of production wafers into the chamber until the quartz side wall temperature is well-above 170 degrees C., but such an expedient involves an unacceptable loss of time as well as loss of materials (i.e., the silicon and quartz scavenging materials inside the reactor chamber). Accordingly there is a need to solve the foregoing problem without loss of production time and without waste of chamber materials.